In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh, papermaking fabric, or wire and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal, system. Papermaking devices well known in the art are described for example in Handbook for Pulp & Paper Technologists 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and Pulp and Paper Manufacture Vol III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. No. 5,539,634 to He, U.S. Pat. No. 5,022,966 to Hu, U.S. Pat. No. 4,982,334 to Balakrishnan, U.S. Pat. No. 4,786,817 to Boissevain et al., and U.S. Pat. No. 4,767,935 to Anderson et al.
In the art of modem high-speed papermaking, it is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include fiber orientation (FO), basis weight, moisture content, and sheet caliper, i.e., thickness. The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to disturbances in the manufacturing process. The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge.
Fiber orientation in papermaking refers to the preferential orientation of the individual fibers on the web. Because of flow patterns in the headbox and the jet impingement on the wire, fibers have a tendency to align in the machine direction (MD) versus other directions in the web. If all of the fibers in the web were perfectly distributed, the paper sheet would have the same properties in all directions. This is called an isotropic sheet and its fiber distribution can be plotted on a polar graph in the form of a circle.
If there are more fibers in one direction than in other directions the fibers are distributed non-uniformly and the sheet is anisotropic. As shown in FIG. 22, the anisotropic fiber distribution can be plotted on a polar graph as a symmetrical ellipse-like geometric 2. An anisotropic sheet has a fiber ratio greater than one and with higher fiber ratios the polar distribution tends to be in the shape of a figure eight. The fiber ratio (anisotropy) is defined as the ratio of maximum to minimum distribution, 90° apart. An isotropic sheet has a fiber ratio of one. The fiber angle α is defined as the angle of the major axis 6 of the ellipse 2 to the machine direction 4. The minor axis 8 is perpendicular to the major axis 6. FIG. 23 also illustrates the definitions of FO ratio (the ratio of max 3 to min 5) and FO angle of fiber distribution in a paper sheet. Fiber ratios can also be defined for other orthogonal directions, and it is common in papermaking to use the ratio of the fiber distribution in the machine direction 4 to the fiber distribution in the cross-machine direction 9.
Fiber orientation in formed webs can influence numerous properties of the final product. In particular, if the fiber orientation distribution is incorrect, then dimensional instability in the form of twist, curl, and skew will occur, and strength axes will not correspond to manufacture axes. This leads to defective products such as paper that jams in printers/copiers, packaging that jams in discrete item containers, and boxes which lean or collapse when stacked. By accurately measuring the fiber orientation on-line in the manufacturing process, it is possible to rectify problems in a timely manner either by manual intervention or by a fiber orientation control system.
Numerous techniques for measuring fiber orientation have been suggested some of which are based on the transmission of laser or maser spots from polarized or unpolarized light sources. The distortion of the spot in transmission through the web or the directional variation in intensity of reflection of the illuminated spot, specular or aspecular, is measured. Because the spot illumination area is relatively small, these techniques do not necessarily yield representative measurements for the sheet. Many of these indirect techniques that measure proxies of fiber orientation are based on the physical principle that fibers scatter more light across their alignment direction than along it.
For example, CA 2,012,351 to Karasikov et al. discloses a system for determining fiber orientation in a stationary or moving web of fibers wherein a small circular light spot is focused onto a first surface of the web thereby forming an ellipse-shaped spot on the opposite or second surface of the web. The elliptical light spot is focused onto an array of light-sensitive elements that are positioned parallel and at a predetermined distance on the second surface of the web. The fiber orientation is determined by evaluating the size, orientation and aspect ratio of the ellipse-shaped spot in the image.
U.S. Pat. No. 4,955,720 to Blecha et al. discloses an on-line method that illuminates one side of a moving sheet with a circular spot of coherent light and acquires a freeze-frame image of the transmitted spot on the opposite side. The fiber orientation angle is estimated from the shape of the transmitted spot, which is presumed to be elliptical.
Similarly, U.S. Patent No. Application 2003/0156293 to Kazuhiko et al. discloses a method that uses an unpolarized focused light beam to illuminate a circular spot on one side of a sheet and images the transmitted spot on the opposite side. Fiber orientation angle and anisotropy are estimated by approximating the transmitted spot shape with an ellipse.
All of the abovementioned methods illuminate the sheet with a circular spot of incident light and their measurement principle requires that the spot of excident transmitted light be elliptical in shape. In fact, the excident spot of transmitted light is elliptical in shape only if the fiber orientation distribution of the sheet has particular properties, such as being unimodal and being symmetric around its maximum. A unimodal distribution has a single maximum and a single minimum. In some instances, the fiber orientation distribution can be bimodal or multimodal, such that there are a plurality of maxima and a plurality of minima in the fiber orientation distribution. Moreover, even if the fiber orientation distribution is unimodal, the distribution of angles is not always symmetric around that maximum, in which case the excident light spot is not elliptical in shape. The abovementioned methods produce unreliable estimates of the fiber orientation angle and the fiber orientation anisotropy, if the fiber orientation distribution of the sheet is not unimodal or is not symmetric.
Multimodal or asymmetric distributions can arise, for example, in multi-ply paperboard, which is made by splicing together two or more separately formed sheets which have different fiber orientation distributions. They can also arise in single-ply sheets, as a result of local vortices in the jet from the headbox, or as a result of other structured differences in the flow field through the jet, especially when the slice channel of the headbox is equipped with vanes separating the flow into layers.
An asymmetric distribution can be unimodal or multimodal, and a multimodal distribution can be symmetric or asymmetric. Detection and quantification of asymmetry or multimodality of the fiber orientation distribution is important in monitoring the process and diagnosing defects in the manufactured product.
DE 3,413,558 to Hartig describes a technique that employs polarized laser light to illuminate a laser spot on one side of a sheet. Four photodiodes are positioned at the nominal edges of the expected excident spot position along x and y axes on the opposite side. The fiber orientation and anisotropy are determined from the ratio of transmitted intensities summed on each axis. As in the above systems, the Hartig device also measures the total or average fiber orientation in the sheet.
U.S. Pat. No. 5,475,233 to Fukuoka et al., U.S. Pat. No. 5,640,244 to Hellstrom et al., and U.S. Pat. No. 6,643,022 to Komppa disclose various methods in which laser light is obliquely directed onto a sheet and the intensity of aspecularly reflected laser light is measured at various directions and inclination angles. The surface fiber orientation determination is based on the differences in the illumination reflectivity when measured from a number of directions. The methods disclosed differ to some extent in the geometries of the illuminations employed.
Due to the low incidence angles used for illumination in these methods, their measurements of reflected light provide information about the fiber orientation distribution in a thin layer at the surface of the sheet. The measurement predominantly represents the first layer of fibers, with reflections mostly occurring at the fiber surfaces facing the sheet surface, and typically lying within 20 microns of the sheet surface. These measurements therefore provide little or no information concerning fiber orientation deeper within the sheet, which can be of great importance in papermaking. Moreover, they can produce biased results if the sheet surface is unconsolidated such that there are fibers partly protruding from the sheet, or if the sheet surface roughness has high amplitude at millimeter scales. They are also sensitive to small changes in the plane of the paper being measured, both in position with respect to the measuring device and in the geometrical relation of the paper plane and the various directions used in illumination and detection of light.
Image analysis is a standard laboratory technique for fiber orientation measurements of paper whereby transmission images of stationary sheets taken from flatbed scanners or similar devices are analyzed. Since paper strongly scatters light, the samples usually must be peeled into layers for transmission or reflection imaging to be feasible. The layers typically are very thin and weigh just a few grams per square meter (gsm). This laboratory process is labor-intensive and not applicable to on-line measurements of moving webs.
Therefore, despite the asserted advantages associated with these fiber orientation measurement systems, none of these apparatuses affords a simple, robust, and accurate device for on-line fiber orientation measurements of a moving web or sheet made of nonwoven components.